In the centrifuging of suspensions, especially in so-called sedimentation centrifuges, it is necessary to remove a layer of liquid which forms upon the centrifuge "cake" which consists of the solids. The removal of this liquid layer must be carried out with regularity and can be effected by dipping a liquid-paring tube or scoop into the liquid layer.
Reference may be had to German patent documents DE-OS 26 03 610, DE-PS 656 125, Swiss patent 205 508, and U.S. Pat. Nos. 3,703,983, 4,900,453, corresponding to German patent document DE-OS 37 26 227 and 4,101,421 corresponding to German patent document DE-OS 26 03 610.
In such systems in which the liquid-paring tube or scoop is moved through the surface of the layer of the liquid to draw off liquid from this layer, there is the problem of establishing that the scoop is deep enough in the contents of the centrifuge drum to draw off the liquid and, nevertheless, does not penetrate significantly into the layer of solids disposed outwardly of the layer of liquid and is withdrawn or retracted when this layer of solids is reached.
This has been achieved in the past primarily by providing a fixed abutment for the movable scoop so that at a given point in its travel it will be stopped, and if desired, swung outwardly again awaiting the next cycle of liquid removal or the next introduction of the charge into the separation centrifuge. The problem with a fixed abutment is that generally all of the liquid of the liquid layer cannot be withdrawn with such a system.
Furthermore, the residence time of a suspension in the drum is not normally constant and can vary and thus it is difficult to control the scoop by timing alone in accordance with predetermined patterns of separation. The reason for this is that conditions within the drum vary with different solids concentrations in the suspensions of different charges, with changes in the amounts of the charges upon successive operations, with viscosity changes of the liquid and with shifts of the filter elements which may be used.
Because of these different effects which cannot be predicted in many cases and cannot therefore be taken into consideration, the operation of the scoop based upon timing considerations is difficult or impractical.